1. Field of the Invention
The present invention relates to a semiconductor device having a circuit comprised of a thin film transistor (hereafter referred to as TFT), and a manufacturing method thereof. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel, and to electronic equipment in which the electro-optical device is installed as a part. In particular, the present invention relates to a dry etching method of etching a metallic thin film, and to a semiconductor device provided with a tapered shape wiring obtained by the dry etching method.
Note that throughout this specification, the term semiconductor device denotes a general device which functions by utilizing semiconductor characteristics, and that electro-optical devices, semiconductor circuits, and electronic equipments are all semiconductor devices.
2. Description of the Related Art
Techniques of structuring a thin film transistor (TFT) using a semiconductor thin film (having a thickness on the order of several nm to several hundred of nm) formed on a substrate having an insulating surface have been in the spotlight in recent years. Thin film transistors are widely applied to electronic devices such as an IC and an electro-optic device, and in particular, development of the TFT as a switching element of an image display device is proceeding rapidly.
Conventionally, Al is often used in a TFT wiring material due to things such as its ease of workability, its electrical resistivity, and its chemical resistance. However, when using Al in a TFT wiring, the formation of a protuberance such as a hillock or a whisker due to heat treatment, and the diffusion of aluminum atoms into a channel forming region, causes poor TFT operation and a reduction of TFT characteristics. High heat resistance tungsten (W), with a relatively low bulk resistivity of 5.5 μΩ·cm, can therefore be given as a preferable wiring material other than Al as a wiring material.
Further, in recent years, the demands of microfabrication techniques have become severe. In particular, with changes in high definition and large screens of a liquid crystal display, high selectivity in the wiring processing step as well as extremely strict control of line width is required.
A general wiring process can be performed by wet etching using a solution or by dry etching using a gas. However, when considering miniaturization of the wiring, maintenance of repeatability, reduction of waste, and decrease of cost, wet etching is unfavorable, and therefore dry etching is considered favorable for wiring processing.
When processing tungsten (W) by dry etching, a mixed gas of SF6 and Cl2 is generally used as an etching gas. While microfabrication with a large etching rate in a short time is possible when this gas mixture is used, it is difficult to obtain a desirable tapered shape. In order to improve the coverage of a lamination film formed on the wiring, there are cases in which the cross section of the wiring is made an intentional forward taper, depending upon the device structure.